In optical dense wavelength division multiplexed (DWDM) systems, such as long distance telecommunication links, it is important to monitor the signal to noise ratio (SNR) for a number of reasons pertaining to the overall performance of the telecommunication links.
The optical signal in these systems is a composite of several wavelengths, determined by International Telecommunications Union (ITU) standards. Each wavelength carries data at a high data rate. Signals weaken as they propagate through long distance optical fibers. To maintain the integrity of the signals, optical amplifiers are placed along the fibers to restore the signal levels. The optical amplifiers add random optical noise to the signal due to the amplified spontaneous emission (ASE) that naturally occurs within these amplifiers. When the light arrives at a receiving site, it is first de-multiplexed, i.e., the wavelength carriers are channeled to different receivers by an optical demultiplexer. Some ASE necessarily falls on the receivers causing errors to occur in the data transmission. As an example, an SNR of 20 dB may be required at a data rate of 2.5 Gb/s to achieve an error rate of one error every 10.sup.15 bits or a bit error rate (BER) of 10.sup.-15. The optical SNR has been used as a predictor of the BER, which is an indicator of the quality of an optical transmission channel.
The small spacing of adjacent optical wavelengths, 0.8 nm, combined with the large required SNR, over 20 dB, make it difficult to measure SNR in a fast and reliable manner. SNR measurements can be made using optical spectrum analyzers, like the Hewlett-Packard HP 71450. In this instrument, high optical resolution is achieved by use of a double monochromator. Large dynamic range is provided by using different optical detectors to observe high power and low power signals. The sensitivity of the HP device can also be increased by changing the scanning speed over wavelength. In real monitoring environments, however, the timing for SNR evaluation and decision making is limited to a few milliseconds. In conventional spectrum analyzers, high resolution and sensitivity can be achieved only by compromising measurement speed.
Another approach to high resolution and sensitivity is possible through the use of tunable optical filters like Fabry-Perot tunable filters. These devices consist of a pair of parallel mirrors with variable spacing which transmit light resonantly when the spacing is equal to an integer multiple of the wavelength. Fabry-Perot filters with high resolution are limited in free spectral range. Another problem with Fabry-Perot filters is their periodic resonance which makes it difficult to uniquely determine the wavelength of the system especially in the presence of multiple wavelengths.
It follows that a need exists for an SNR monitor capable of resolving ITU wavelengths and measuring the ASE at levels more than 20 dB below the signal level.